This disclosure relates to an acoustic device with passive radiators.
Some acoustic devices include passive radiators. For example, U.S. Pat. No. 5,850,460 discloses an acoustic device with passive radiators of the same effective vibration area and the same effective vibration mass disposed in mutual opposition, and driver units of the same effective vibration area and the same effective vibration mass disposed in mutual opposition, all mounted to an enclosure. The vibration-reaction forces of the opposing passive radiators and opposing driver units on the enclosure are thereby mutually cancelled, and enclosure vibrations are thus reduced. Powerful bass output can be achieved because the diameter of the passive radiators can be increased at will and the use of two passive radiators achieves a large vibration area.
The total mass of the passive radiators needs to be sufficient such that the acoustic device can be tuned to the desired frequency. For bass devices, tuning is usually around 40 Hz. In many cases the mass of one or more of the radiators must be increased by adding weight. Acoustic devices with passive radiators are thus typically relatively heavy, which limits their usefulness in portable products or products in which weight is a concern. Also, with mass-balanced passive acoustic radiators, both radiators are displaced by the same amount. The relatively large excursion of the radiator that carries the active transducer increases the intermodulation distortion which can result in audible unwanted sounds.